The BY-CD28/CTLA-4 co-stimulatory pathway has a critical role in T cell activation, differentiation and tolerance. For this reason, methods directed toward manipulating this pathway have received considerable attention for the treatment of autoimmunity, and appear to have great therapeutic potential. Our finding that B7-1 on T cells influences IL-4 production, together with the discovery of the CD28 homologous ICOS, which appears to be particularly important for IL-10 production, have raised new questions about how co-stimulation regulates the responses of elf-reactive T cells. The regulation of IL-4 by b7-1 on T cells suggests that B7-1 on T cells may regulate the differentiation and encephalitogenic potential of self-reactive T cells. Because IL-10 is a critical regulator of autoimmune disease, engagement of ICOS may have important influences on the initiation and progress of autoimmune diseases. In this proposal we will investigate the role of B7 and ICOS-mediated signals in both the afferent arm of the autoimmune response where T cells are driven to mediate tissue destruction: 1. We will investigate the role of ICOS in the induction of autoimmune responses in the CNS. We will examine how anti-ICOS mAbs and ICOS-Ig affect the initiation of EAE and determine whether ICOS deficient mice are susceptible to EAE. 2. We will analyze the role of ICOS in the function of encephalitogenic and regulatory T cells. We will use ICOS deficient ICOS deficient mice to study to determine whether ICOS provides co-stimulatory signals needed by regulatory T cells. 3. We will investigate the role of B7 on T cells during the induction and effector phases of EAE. Our B7 deficient strains are the definitive tools for examining B7 on T cells regulates the pathogenic potential of naive self reactive T cells and/or affects the function of activated of activated self reactive T cells. We also investigate how interactions between ICOS and the B7-CD28/CTLA-4 pathway affect the initiation and progression of autoimmune disease. In collaboration with Project 2, we will use MOG, 35-55 specific TCR transgenic mice to visualize the effects of ICOS dysregulation on the activation, migration and expansion of naive and activated T cells in vivo. We will also generate B7 deficient, MOG TCR transgenic and ICOS deficient, MOG TCR transgenic strains as a tools for studying the responses of B7-/- T cells and ICOS-/- T cells and ICOS-/- T cells in vivo during the induction and effector phases of EAE. Taken together, these studies should lead to new insight into how the B7/CD28/CTLA-4 and ICOS pathway regulate the responses of self-reactive T cells, and hopefully will facilitate manipulation of these pathways to effectively treat autoimmune disease.